1. Field of the Invention
The present invention relates to a seat adjustment device for adjusting the front and rear positions of seats used in vehicles and, in particular, to an improved seat adjustment device including dual locking mechanisms, that is a dual-side locking type adjustment device.
2. Description of the Prior Art
A conventional seat adjustment device typically comprises a pair of rail assemblies such that each assembly includes a fixed-side rail adapted to be fixed to the floor of a vehicle body and a movable-side rail adapted to be mounted to a seat. Two two rails are thus engaged with each other so that one rail is slidable relative to the other rail. Conventional seat adjustment device can be classified into two types. One type is referred to as a single-side locking type in which a lock mechanism for locking the fixed-side and movable-side rails is provided only in one of the two assemblies. The other type is called a dual-side locking type in which there are provided dual lock mechanisms in both of the assemblies respectively. In the single-side locking system, since both rails of the other rail assembly are free to slide relative to each other, in case of sudden stops or the like, pushing forces may be biased onto the seat to deform a seat frame and thus it is necessary to increase the rigidity of the seat frame by attaching a reinforcement member thereto or by other suitable means. A seat employing this type of adjustment device is thus increased in weight and also becomes more expensive. For these reasons as well (as from the viewpoint of safety), the dual-side locking system has been employed increasingly in these years. In the both-side locking system employed in the seat adjustment device, however, both lock mechanisms for the respective rail assemblies must perform their respective locking/unlocking operations cooperatively and simultaneously with each other, which requires components of higher precision.
Specifically, the lock mechanism provided in the above both-side locking system is constructed such that engagement bores are formed at required intervals in the fixed-side rail in the longitudinal direction thereof. A lock plate provided with corresponding click pieces to be engaged into the engagement is mounted to the movable-side rail wherein the lock plate is movably operated to engage the click pieces into the engagement bores and disengage the former out of the latter. In such a manner, the movable-side rail can be locked and unlocked relative to the fixed-side rail. In the case of the dual-side locking system, such locking and unlocking operations of both lower rails must be carried out simultaneously, and the lock mechanisms require more accurately formed engagement bores in the fixed-side rail, lock plate and seat frame, as well as more time and labor for positioning them during assembly.